PieceTable: newline test from the line index, memchr rebuild; remove dead code

GetLine and LineColToByteOffset fetched each line's last byte (a piece
lookup, an allocation and a cache write) to see whether it was a newline;
the line index already says so: every line but the last ends at one.
LineColToByteOffset runs on every insert. RebuildLineIndex scans with
memchr instead of byte by byte.

Removed, having no callers: OptimizedSearch (built only into its own
test), UndoNodePool.h, SwapManager::RecordSplit/RecordJoin (replay still
reads SPLIT/JOIN), PieceTable::Reserve/Capacity and its capacity
constructor, Buffer's write-only render column, Editor's mode, no-kill,
dirtyex and repeatable flags and the UCount getter, and
RetryPolicy::Conservative.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

Swap.h

@@ -24,6 +24,7 @@ namespace kte {
 enum class SwapRecType : std::uint8_t {
 	INS = 1,
 	DEL = 2,
+	// SPLIT and JOIN are no longer written; replay still reads them.
 	SPLIT = 3,
 	JOIN = 4,
 	META = 0xF0,
@@ -213,10 +214,6 @@ private:
 
 	void RecordDelete(Buffer &buf, int row, int col, std::size_t len);
 
-	void RecordSplit(Buffer &buf, int row, int col);
-
-	void RecordJoin(Buffer &buf, int row);
-
 	void RecordCheckpoint(Buffer &buf, bool urgent_flush);
 
 	void maybe_request_checkpoint(Buffer &buf, std::size_t approx_edit_bytes);